Together with 18544e06, updating the existing
migration is safe, because

 a) if the migration failed somewhere due to the multiple leaf nodes, it was
    never applied, and changing it is safe;
 b) if it succeeded, it will be renamed to 0008_ later, and thus
    re-applied

Add additional rules&permissions in order to differentiate different access to individual timetables

Close #70

cancelled_for_teachers, comment

- timefrom/timeto from Event
- periodfrom from Exam